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Roofing may seem complicated but is easy if you follow some basic rules. Many roofs are now made from trusses which takes some of the thinking out of the work. It is still important to know how the "cut" a roof as it helps you understand roofing better and in houses with pergolas or exposed cathedral ceiling you have to be able to cut roofs.
Shown here is a roofing boat. This is cut to the angle of the roof and used to cut plumb cuts and birdmouths on rafters. The top piece is always cut to 100mm width as this corresponds to measurements given in roofing tables. These are marked on the top of the boat.
Shown here is a pattern rafter (the top shows a plumb cut and the bottom a birdsmouth). This is the rafter that is traced to make the other "commons" which make up the majority of most roofs. The rafters are all nailed into a ridge and the top wall plate.
You work out the length of a rafter by calculating the length based on tables in the roofing book. All you need to know in the overall span of the roof and form this work out the "run" of the rafter.
To work out the RUN based on 1000mm span.
1/2 the span (500mm) and
then substract 1/2 the width of the ridge (50mm in this example, so 25mm).
500mm - 25mm = 475mm RUN
We mark out the positions of the rafter and cut the commons. Once these are ready when set up the ridge and nail the rafters in place. Depending on the roof types we may need to cut hips, valleys and creepers.
Note this roof has been cut, the one below is trusses.
Trusses are engineered roof components prefabricated in a factory. The roof loads and designs are worked out via a software program. Advantages of using trusses are that they are quick to asemble, easier than putting up a cut roof, can span large distances and can use smaller size timber members thus reducing costs. Trusses can be joined up to exisitng cut roofs without too much trouble.
Shown here is a roofing boat. This is cut to the angle of the roof and used to cut plumb cuts and birdmouths on rafters. The top piece is always cut to 100mm width as this corresponds to measurements given in roofing tables. These are marked on the top of the boat.
You work out the length of a rafter by calculating the length based on tables in the roofing book. All you need to know in the overall span of the roof and form this work out the "run" of the rafter.
To work out the RUN based on 1000mm span.
1/2 the span (500mm) and
then substract 1/2 the width of the ridge (50mm in this example, so 25mm).
500mm - 25mm = 475mm RUN
Note this roof has been cut, the one below is trusses.